This invention relates to a tungsten (W) wire useful as the filament material for lamp, more particularly to a W wire which is useful as the wire when preparing a filament for, for example, a halogen lamp and which will not bring about any deformation or wire breaking of the filament not only during lighting at a high temperature above the filament temperature of incandescent lamp but also during use under severe vibration conditions.
A filament for lamp to be used under severe vibration conditions, for example, a filament for a halogen lamp is constituted typically of a doped W wire with high vibration-proof characteristic. Such a doped W wire is prepared as outlined below.
That is, first, WO.sub.3 powder with a predetermined particle size distribution is formulated with dopants as represented by K, Si and Al. Then, the powder is subjected to reducing treatment in a hydrogen furnace to form W powder having the dopants carried thereon. Subsequently, the W powder is pressure molded into a green compact.
The green compact is pre-sintered at a temperature of, for example, about 1200 .degree. C., and current passage sintering is effected with the both ends thereof being used as terminals to form a sintered bar. The sintered bar is generally subjected to swaging, during which step the recrystallization heat treatment is applied thereto, followed further by drawing, to form a wire with a predetermined wire diameter.
In the series of processes, the dopant contained in the green compact behaves as described below.
First, in the sintering process, sintering which occurs between W powders proceeds, whereby crystal grains of W grow, and at the same time, the dopants are pyrolyzed with a part thereof being vaporized. In the sintered bar on completion of sintering, the dopants exist in a large number of small spherical dope pores or sintering pores.
Then, when sintered bar is swaged, the above-mentioned W crystal grains become a fibrous structure elongated in the wire axis direction, and at the same time the dope pore is deformed into a slender pore. When working is further progressed, the dope pore is gradually flattened in the wire axis direction.
Subsequently when the wire is subjected to secondary recrystallization treatment by heating at a high temperature (e.g. lamp flashing), the dopant is vaporized and, the fine bubbles arrayed with a certain length are formed in the wire axis direction.
Through the effect on a large number of these arrayed bubbles dispersed in the wire axis direction, growth of the recrystallized grains in the direction perpendicular to the wire axis direction is inhibited, and as a consequence, the growth of the recrystallized grains proceed selectively in the wire axis direction, whereby greatly lengthy recrystallized grains elongating in the wire axis direction is formed, and these are interlocked each other to improve deformation-proof property of the wire at a high temperature.
Shortly speaking, the dispersion mode of the arrayed bubbles affects grain growth during recrystallization of the wire, thereby affecting significantly the vibration-proof property, and the deformation-proof property at high temperatures.
Recently, halogen lamps have been used in various illumination fields, and the use environment is becoming more and more severe accompanied therewith. Under such circumstances, in the filament of the doped W wire commercially available so far, vibration-proof properties on high temperature lighting is insufficient, and the problems of deformation of the filament during lighting, further nonuniformity of luminous intensity distribution have been pointed out, and there is an increasing demand for development of a doped W wire with excellent vibration-proof property on lighting as well as with high reliability.
The present invention has been developed in order to respond to such demand, and its object is to provide a doped W wire excellent in vibration-proof property at high temperature.